Body composition is the distribution of body weight between lean body mass and body fat. Body composition analysis is used to define the condition of obesity, and provides useful diagnostic data for the treatment of a number of chronic conditions such as diabetes mellitus. Moreover, body composition analysis provides a method for monitoring the effects of diet and exercise.
Hydrostatic weighing has been the method of choice for assessing body composition. The subject is weighed while submerged in a tank of water, after expelling as much air from the lungs as possible. Comparison of the weight of the subject while immersed to the weight of the subject in air allows determination of body density without the complex step of measuring the amount of water displaced. After determination of the density of the body of the subject, the percentage of fat is determined by assuming constant densities of fat mass (0.9 g/cc) and lean mass (1.1 g/cc). Although considered quite accurate, the hydrostatic method of assessing body composition is burdensome, expensive, and inconvenient, and frequently cannot be applied to elderly, weak, or ill subjects.
The method of assessing body composition most often used by physicians in office settings has been measurement of skin fold thickness with calipers. Even when performed by a skilled practitioner, and after averaging the values from, for example, three measurements at each of seven different skin fold sites, the repeatability and reliability of caliper results is not good and the procedure itself is frequently uncomfortable for the subject.
Other proposed techniques of assessing body composition have included isotope dilution, near infra-red and dual energy x-ray imaging, and measurement of fat-soluble gases; each of these involve cost and/or accuracy issues not yet satisfied.
An indirect method for estimating body composition based on the measurement of electrical impedance across a portion of the subject's body has become widespread and accepted. Electrodes are effectively connected to the subject's body at two separated points and a low-voltage, high-frequency alternating current signal is passed through a circuit including that portion of the subject's body. The impedance of the body is measured and used to provide an indication of body composition, as follows.
Electrical conduction in the human body is related to the relative quantity of water and electrolytes available to carry the signal. Lean body mass (including muscle, bone, cartilage, tendons and the like) is about 75% water, generally rich in conductive ions, while fat is only about 3 to 7% water. Consequently the electrical conductivity of the lean mass is much greater than that of fat; accordingly, measurement of the body's overall impedance provides an indication of the relative ratio of the fat and lean constituents, and thus of the body composition.
Conventionally, the measured impedance is used as input to one or a set of regression equations derived from a data base of impedance values measured with subjects of known body composition, typically having been measured using the hydrostatic densitometry technique discussed above. The regression equations generally include weight and height as variables. More sophisticated models may also include gender. Less commonly, body type or morphology is included. The measured value for the impedance is also dependent on the circuit path through the body, as defined by the placement of the electrode contacts used to introduce the current and to measure the impedance. Most commonly these electrode sites have been near the wrist on one hand and near the ankle on one foot, although other electrode placements have been used.
Prior art describing variations on this basic technique include U.S. Pat. Nos. 4,947,862 to Kelly, U.S. Pat. No. 5,449,000 to Libke et al, and U.S. Pat. No. 5,720,296 to Cha. These references all show wrist-to-ankle current pathways; accurate measurements require the subject to have fasted for a set length of time prior to measurement, to have voided shortly prior, and to assume a prone or supine position with at least one foot exposed. These inconveniences are necessary for accurate measurement of impedance because fluid anomalies in the abdomen can cause false readings. Obviously the inconvenience of such constraints greatly limits the use of the technique and the market for devices implementing the technique.
U.S. Pat. No. 5,611,351 assigned to the Tanita Corporation discloses a body circuit passing from one foot to the other while the subject stands on a weighing scale. Although this overcomes some of the problems associated with wrist-to-ankle measurement, the measured impedance is still subject to lower abdomen fluid concentration anomalies and the difficulty of short circuiting if the subject's thighs touch.
Finally, in U.S. Pat. Nos. 5,579,782 and 5,817,031, both assigned to the Omron Corporation, a hand-held bio-electric impedance device is described wherein the subject grasps a portable impedance analyzer that measures hand-to-hand impedance, which is then related to body composition.